Rf/optical shared aperture for high availability wideband communication rf/fso links

ABSTRACT

An RF/Optical shared aperture is capable of transmitting and receiving optical signals and RF signals simultaneously. This technology enables compact wide bandwidth communications systems with 100% availability in clear air turbulence, rain and fog. The functions of an optical telescope and an RF reflector antenna are combined into a single compact package by installing an RF feed at either of the focal points of a modified Gregorian telescope.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/076,885, titled “RF/Optical Shared Aperture for High AvailabilityWideband Communication RF/FSO Links,” filed Mar. 31, 2011, incorporatedherein by reference. U.S. patent application Ser. No. 13/076,885 claimsthe benefit of U.S. Provisional Patent Application No. 61/320,017,titled “RF/Optical Shared Aperture for High Availability WidebandCommunication RF/FSO Links,” filed Apr. 1, 2010, incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and morespecifically, it relates to radio-frequency (RF) and free-space optical(FSO) communications.

2. Description of Related Art

The availability of free-space optical links is limited by atmosphericsand weather conditions. RF and microwave systems are less susceptible tothese problems. Hybrid RF/Optical communications systems that can switchback and forth between RF and Optical transmissions in order to optimizethe overall availability of the link and maximize communicationperformance would be a solution. The use of a terminal with sharedRF/Optical aperture could provide the overall link availabilityadvantage described above in the smallest form factor possible. Thisbecomes increasingly important in deployment scenarios where space is ata premium and the use of multiple RF and optical apertures is notdesirable, for example on small aircraft, satellites, and in certainground vehicles and ground deployments.

As discussed in U.S. Pat. No. 6,667,831, incorporated herein byreference, FIG. 1 illustrates a traditional Gregorian telescope 100according to the prior art. The Gregorian telescope 100 has a concaveprimary mirror 102 and a concave secondary mirror 104. In manytraditional Gregorian telescopes, the primary mirror has a paraboliccurvature and the secondary mirror has an elliptical curvature. Thesecondary mirror 104 is disposed outside the focal plane of the primarymirror 102, and the mirrors share a common optical axis 106. The primarymirror 102 reflects light from a far field and directs the light towardsthe secondary mirror 104. The secondary mirror 104 is appropriatelysized and positioned so that light reflecting off the primary mirror 102is incident on the secondary mirror 104. The secondary mirror 104reflects light and directs it through an aperture 108 in the primarymirror 102 that is centered about the optical axis 106. The light isthereafter imaged at the focal plane 110 of the compact telescope foradvantageous use.

FIG. 2 illustrates an embodiment of a compact telescope as discussed inU.S. Pat. No. 6,667,831. The compact telescope 200 comprises a firstreflecting surface 202 and a radially defined second reflecting surface204. The first reflecting surface 202 includes an annular outer portion206, a radially defined inner portion 208, and a radially definedaperture 210. Other shapes may be used for these elements of the compacttelescope, however, alternative shapes may increase the complexity ofthe optics.

The outer portion 206 of the first reflecting surface 202 is thefunctional equivalent of the primary mirror in a traditional Gregoriantelescope, while the inner portion 208 is the functional equivalent ofthe secondary mirror. Therefore, hereinafter, the term “primary mirror,”as it relates to a compact telescope, is used interchangeably with theouter portion 206 of the first reflecting surface. Likewise, the term“secondary mirror”, as it relates to a compact telescope, is usedinterchangeably with the inner portion 208 of the first reflectingsurface. The primary and secondary mirrors 206, 208 are both concave,with the curvature of the secondary mirror 208 being greater than thecurvature of the primary mirror 206. In FIG. 2, both the primary mirror206 and the secondary mirror 208 have elliptical curvatures (i.e., conicbetween −1 and 0). Those skilled in the art will recognize that withboth mirrors having elliptical curvatures, correcting for both sphericaland coma aberrations is facilitated without the need for additionaloptical elements. In an alternative embodiment, the primary mirror 206may have a parabolic curvature (i.e., conic equal to −1) and thesecondary mirror 208 may have an elliptical curvature. Other curvaturesmay also be used for the primary and secondary mirrors 206, 208 of thecompact telescope.

The optical axes 212 of the primary and secondary mirrors 206, 208 arecoincidental. Additionally, the aperture 210 and the second reflectingsurface 204 are centered upon the coincident optical axes 212.Non-coincidental and/or off-axis optics may be employed, however,coincident optical axes reduce complications in aligning the opticalelements and simplify the optics of the compact telescope.

In the embodiment of FIG. 2, the primary and secondary mirrors 206, 208form the integral first reflecting surface 202. Such a double-curvedmirror facilitates manufacturing and optical axis alignment of eachcurvature on the first reflecting surface 202. This is important becausegreater errors in axis alignment result in greater optical aberrations.For example, a double-curved mirror may be Manufactured using diamondturning or other appropriate equipment that is frequently used to createhigh quality mirrors. With the appropriate manufacturing equipment, theprimary and secondary mirrors may be manufactured sequentially using asingle piece of equipment without realigning the equipment to obtaincoincidental axes.

Alternatively, in lieu of a double curved mirror, the compact telescopemay comprise a first reflecting surface having an annular shape (theprimary mirror), with a third reflecting surface (the secondary mirror)disposed within the inner radius of the first reflecting surface. Thecurvatures of this alternative embodiment for the first and thirdreflecting surfaces are the same as the curvatures for theaforementioned outer and inner portions, respectively.

Returning to FIG. 2, the second reflecting surface 204 is a planarsurface, hereinafter referred to as the “folding mirror”. The foldingmirror 204 optically couples the primary mirror 206 to the secondarymirror 208. The folding mirror 204 is disposed between the firstreflecting surface 202 and the focal plane of the primary mirror 206.Thus, light from a far field may enter the primary aperture of thecompact telescope 200 and reflect off the primary mirror 206 towards thefolding mirror 204. The folding mirror 204 reflects such light towardsthe secondary mirror 208, and the secondary mirror 208 reflects thelight back towards the folding mirror 204. Upon this second reflectionfrom the folding mirror 204, the light passes through the aperture 210.Light emerging from the aperture 210 creates an upright image at thefocal plane 214 of the compact telescope that may be advantageouslyused.

Alternative embodiments of the compact telescope may include a curvedfolding mirror. A curved folding mirror preferably has a high radius ofcurvature, such as a radius of 1 meter or more. Smaller curvatures mayalso be employed. In another alternative embodiment, the folding mirrorcomprises a steering mirror. The steering mirror may have a planar orcurved reflective surface. A steering mirror having a curved reflectivesurface may help improve the optics of a compact telescope when theoptical axes of the primary and secondary mirrors are impreciselyaligned.

SUMMARY OF THE INVENTION

It is an object of the present invention to combine free space opticaland RF communications into a single networked system to provide compact,robust, high bandwidth mobile communications for commercial, militaryand government applications.

This and other objects will be apparent based on the disclosure herein.

The invention provides embodiments of an RF/Optical shared aperture forhigh availability wideband communication RF/FSO links that have acommunications terminal capable of transmitting and receiving opticalsignals and RF signals simultaneously. RF/Optical hybrid technologyenables communications systems with 100% availability in clear airturbulence, rain and fog. RF and optical communications transmittershave different sensitivities to atmospheric perturbations; consequentlyselective switching between systems provides an optimal solution. Inembodiments of the current invention, a RF/millimeter wave (RF/MM) beamis fed into an optical telescope at one of two focal points in a mannerthat does not perturb the optical beam. In some embodiments, a planarpatch fixed beam array antenna with a through hole in the array centeris placed at the co-location of the optic focal plane of a compacttelescope to create the optical and radio frequency (RF) shared apertureterminal. The patch array antenna is the feed for RF frequency. Thisfeed is designed and configured so that it does not disturb the path ofthe original optical rays. Both optical and RF share the same aperturewithout interfering with the performance of one another. In otherembodiments, a RF/millimeter-wave feed is installed at the focal pointof the primary reflector, which is in front of the secondary mirror. Aproperly designed dichroic plate replaces the standard aluminumsecondary mirror of the modified Gregorian system. This dichroic platereflects the optical signal and transmits the RF signal. This inventionenables extremely compact implementation of RF/Optical hybrid technologyfor high data capacity and high availability communications.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate embodiments of the invention and, togetherwith the description, serve to explain the principles of the invention.

FIG. 1 shows a prior art Gregorian telescope.

FIG. 2 shows a prior art modified Gregorian telescope.

FIG. 3 shows an embodiment of a modified Gregorian telescope with adichroic folding mirror, an RF signal compensating element, a lightsource at focal point 1 and an RF feed at focal point 2.

FIG. 4 shows a picture of a millimeter-wave scalar feed horn antennawhich can be used in the embodiment of FIG. 3.

FIG. 5 shows an embodiment of a modified Gregorian telescope with analuminum folding mirror and both a light source and an RF antenna atfocal point 1.

FIG. 6 shows a black-and-white line illustration of a 16×10 slot arrayfeed antenna.

FIG. 7 shows the measured antenna radiation pattern of a 16×10 slotarray feed antenna.

DETAILED DESCRIPTION OF THE INVENTION

To achieve a high-availability, high throughput data link, it isadvantageous to employ a hybrid communication system architecturecapable of both RF and optical transmission. This approach enables highcapacity data transfer over the optical channels during good atmosphericand weather conditions and provides a smooth transition to a lower datarate RF channel in bad weather. For applications where limited space isavailable, such as an airborne platform, it is desirable to combine thefunctions of the optical telescope and the RF antenna into a singlecompact package. Such a device, referred to herein as an RF/opticalshared aperture, is a terminal that is capable of transmitting andreceiving optical signals and RF signals simultaneously to and from thesame aperture. The present invention utilizes recent developments inprecision-machined telescopes and customized patch array antenna feeds.See, e.g., U.S. Pat. No. 6,667,831, titled Compact Telescope,”incorporated herein by reference.

Dichroic Folding Mirror and RF Feed

FIG. 3 shows an embodiment of the present invention having an RF feed300 located at the focal point of the primary mirror 306. The embodimentincludes a dichroic folding mirror 304, a primary mirror 306 (shown incross section) and having an inner perimeter 350 and an outer perimeter352, a secondary mirror 308 shown cross section) and disposed withinsaid inner perimeter and having an aperture 310. The embodiment furtherincludes a light source 312 at a second focal plane 314 of the systemand a compensating element 316. If the folding mirror is made ofmaterial, such as solid aluminum, that will not transmit an RF signal,it would block the RF energy that is directed from RF feed 300 towardthe primary and secondary mirrors. In the present invention, the soliddisc-shaped metallic folding mirror 204 of the prior art is replacedwith folding mirror 304 made of dichroic material that is able toefficiently reflect the incident optical beam from light source 312,often having a wavelength of 1550 nm, while maintaining transparency toRF energy. Due to the design of the telescope, the direction of theincident optical beam ranges between normal to the surface and about 40°from normal.

The dichroic folding mirror 304 achieves high reflective performance inthe optic band by acting as synthetic Bragg crystals. This effect iscreated by layers of materials deposited in a periodic stack. Thethickness of each layer is often less than one tenth of a micrometer.The materials used for the layers are dielectrics, such as pyrex.Through constructive interference of the reflected light, this structurecan efficiently reflect the incident optical signal according to Bragslaw, which relates the reflected wavelength to the angle of the incidentlight. The dichroic folding mirror 304 is transparent to the RF signalbecause the thickness of the layers that construct the plate istypically less than three orders of the RF wavelength. This produces nowave phenomena as the RF signal passes through the plate; therefore, theBragg crystal is transparent to the RF signal. There is no constructivereflection. Furthermore, the conductivity of the layer material is verylow in the RF band. The insertion loss is negligible. A portion of theRF signal transmitting through folding mirror 304 will impact onto theprimary mirror 306 and a portion will impact onto the secondary mirror308. Because the secondary mirror has a different curvature than theprimary mirror, the secondary mirror will reflect incident RF signal ina different direction than the direction of RF rays that reflect fromthe primary mirror. In order to compensate the RF signal direction forthe different curvature of the secondary mirror, a dielectric lens 316,having a properly designed thickness and curvature, is inserted betweenthe folding mirror 304 and the RF source 300. As shown in FIG. 3, anembodiment of the invention places dielectric lens element 316 incontact with folding mirror 304. In this embodiment, the diameters ofthe dielectric lens, the folding mirror and the secondary mirror 308 areabout the same. Dielectric lens element 316 has a thickness andcurvature that will cause the transmitted RF signal to reflect from thesecondary mirror 308 such that the signal will have the same directionas the signal reflecting from the primary mirror 306. The RF signalreflecting from the secondary secondary mirror will be collimated as itpropagates out of the telescope. Based on the teachings herein, thoseskilled in the art will be able to properly design dielectric lenselement 316.

FIG. 4 shows a picture of an exemplary millimeter-wave scalar feed hornantenna which can be used as RF feed 300 as depicted in FIG. 3. Thistype of feed horn has a highly symmetrical antenna pattern and very lowside lobe characteristics. It can be designed to uniformly illuminatethe primary mirror, while occupying a small volume (25 mm diameter, 70mm length). Scalar teed horn antennas are commercially available.

Aluminum Secondary Mirror and Back-Mounted RF Feed

FIG. 5 illustrates a device that provides the RF signal and the opticalsignal from the same aperture. Elements common to the embodiment of FIG.3 are identically numbered. The apparatus places a RF feed 420 and thelight source 312 at focal point 1 of FIG. 5. In this configuration, RFenergy that is transmitted through the aperture 310 of the secondarymirror 308, is reflected by aluminum folding mirror 404 and is emittedfrom the primary mirror 306. Since both RF and optical feeds share thisfocal point, the RF feed antenna must be planar and include a centralhole through which the optical beam can pass. A slot-array feed antenna,as known in the art, may be designed to include a central hole, asillustrated in FIG. 6. This slot array design was taken throughmanufacturing at Kyocera America. The actual antenna radiation patternof the Kyocera 16×10 slot-array feed is shot in FIG. 7.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments disclosed were meant only to explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

We claim:
 1. An apparatus, comprising: a first reflecting elementcomprising a concave primary mirror and a concave secondary mirror,wherein said concave primary mirror has an inner perimeter, an outerperimeter and a first curvature, wherein said concave secondary mirroris disposed within said inner perimeter and comprises a central apertureand a second curvature that is greater than said first curvature; afolding mirror facing said concave secondary mirror, wherein saidfolding mirror is configured to reflect a radio-frequency signal (RFS)and an optical signal (OS); an RFS source configured to direct said RFSthrough said central aperture such that said RSF will first be reflectedby said folding mirror and will then be reflected by said concaveprimary mirror; and an OS source configured to direct said OS throughsaid central aperture such that said OS will first be reflected by saidfolding mirror and will then be reflected by said concave primarymirror.
 2. The apparatus of claim 1, wherein said OS comprises awavelength of 1550 nm.
 3. The apparatus of claim 1, wherein said foldingmirror, said primary mirror and said secondary mirror all comprisematerial that is reflective to both said RFS and said OS.
 4. Theapparatus of claim 1, wherein said RFS source comprises a slot-arrayfeed antenna.
 5. The apparatus of claim 4, wherein said slot-array feedantenna comprises a central hole, wherein said central hole ispositioned between said OS source and said central aperture such thatsaid RFS will propagate through said central hole and said centralaperture to impact said folding mirror.
 6. A method, comprising;providing the apparatus of claim 1; producing said RFS from said RFSsource; directing said RFS through said central aperture, wherein saidRSF is reflected by said folding mirror and is then reflected by saidconcave primary mirror; producing said OS from said OS source; anddirecting said OS through said central aperture, wherein said OS isreflected by said folding mirror and is then reflected by said concaveprimary mirror.
 7. The method of claim 6, wherein said OS comprises awavelength of 1550 nm.
 8. The method of claim 6, wherein said foldingmirror, said primary mirror and said secondary mirror all comprisematerial that is reflective to both said RFS and said OS.
 9. The methodof claim 6, wherein said RFS source comprises a slot-array feed antenna.10. The method of claim 9, wherein said slot-array feed antennacomprises a central hole, wherein said central hole is positionedbetween said OS source and said central aperture such that said RFS willpropagate through said central hole and said central aperture to impactsaid folding mirror.
 11. A fabrication method, comprising: providing afirst reflecting element comprising a concave primary mirror and aconcave secondary mirror, wherein said concave primary mirror has aninner perimeter, an outer perimeter and a first curvature, wherein saidconcave secondary mirror is disposed within said inner perimeter andcomprises a central aperture and a second curvature that is greater thansaid first curvature; configuring a folding mirror such that it isfacing said concave secondary mirror, wherein said folding mirror isconfigured to reflect a radio-frequency signal (RFS) and an opticalsignal (OS); configuring an RFS source to direct said RFS through saidcentral aperture such that said RSF will first be reflected by saidfolding mirror and will then be reflected by said concave primarymirror; and configuring an OS source to direct said OS through saidcentral aperture such that said OS will first be reflected by saidfolding mirror and will then be reflected by said concave primarymirror.
 12. The method of claim 11, wherein said folding mirror isconfigured to reflect an OS comprising a wavelength of 1550 nm.
 13. Theapparatus of claim 11, wherein said folding mirror, said primary mirrorand said secondary mirror all comprise material that is reflective toboth said RFS and said OS.
 14. The method of claim 11, wherein said RIPSsource comprises a slot-array feed antenna.
 15. The method of claim 14,wherein said slot-array feed antenna comprises a central hole, whereinsaid central hole is positioned between said OS source and said centralaperture such that said RFS will propagate through said central hole andsaid central aperture to impact said folding mirror.